Why I've reopened FL-6379:

again I had to do a fresh install of a system and again I lost some time to debug trival error with lightdm which is:

(from LightDM gentoo wiki)

To start LightDM on boot, add dbus and xdm to the default runlevel.
dbus is necessary because LightDM depends on it to pass messages

I think that /etc/init.d/xdm should be improved so that lightdm is always started after dbus.

cardinal pointed that dbus is pulled by elogind but in my case so far there is no need to add elogind to any runlevel